This application is related and claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2000-392939, the entirety of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a manufacturing method for a ceramic oscillator formed of a piezoelectric ceramic, and particularly, to a manufacturing method for an energy-confinement type ceramic oscillator which utilizes the thickness extensional vibration mode and in which the polarization process of the mother substrate has been improved.
2. Description of the Related Art
Hitherto, various energy-confinement type ceramic oscillators utilizing the thickness extensional vibration mode have been proposed. This type of ceramic oscillator has been manufactured under the following processes.
First, electrodes are formed entirely over both surfaces of a piezoelectric mother substrate. Next, polarization processing is performed for the piezoelectric mother substrate by applying an electric field on the electrodes on both sides thereof. Then, by etching the electrodes, resonant electrodes in discrete ceramic oscillator units are formed, and the frequency of a single ceramic oscillator is measured on the mother substrate. When the measured frequency deviates from target frequency, a frequency adjustment is performed. Thereafter, the piezoelectric mother substrate is cut into discrete ceramic oscillator units. The ceramic oscillator obtained by cutting is used as a ceramic oscillator as a finished product, as it is. Alternatively, by affixing lead terminals to the ceramic oscillator and by applying an outer packing thereon, a ceramic oscillator as a finished product is obtained.
Next, the frequencies of the obtained ceramic oscillators as finished products are measured, and ceramic oscillators each having a frequency in a predetermined frequency range are selected as conforming articles.
Meanwhile, the frequency fosc of a ceramic oscillator is represented by fosc=N/t (here, N is a frequency constant, and t is the thickness of a piezoelectric substrate). Hence, for the above-mentioned frequency adjustment, there are two known methods: (1) a method for adjusting the thickness of a piezoelectric substrate, and (2) a method for adjusting the above-mentioned frequency constant.
For example, Japanese Unexamined Patent Application Publication No. 6-224677 discloses a method for forming vapor-deposited films on the surfaces of resonant electrodes on a piezoelectric mother substrate, in accordance with the deviation of the measured oscillator frequency from an target oscillation frequency. Japanese Unexamined Patent Application Publication No. 10-190388 discloses a method for increasing the thickness of electrode films by plating the surfaces of the resonant electrodes formed on a mother substrate, in accordance with the above-described frequency deviation. Japanese Unexamined Patent Application Publication No. 7-106892 discloses a method for applying a frequency adjusting ink on resonant electrodes formed on a mother substrate, in accordance with the above-described frequency deviation.
Also, Japanese Unexamined Patent Application Publication No. 758569 discloses a method for adjusting the thickness of a piezoelectric mother substrate by lapping the piezoelectric substrate until the antiresonant frequency corresponding to a desired resonant frequency is attained, while measuring the resonant frequency in the process of working, when lapping the piezoelectric substrate after having polarized it.
On the other hand, the above-mentioned method (2) for adjusting the frequency constant is disclosed in Japanese Unexamined Patent Application Publication No. 7-106893. Herein, the oscillation frequency of piezoelectric resonators each having an outer package applied are measured, and the deviation of the oscillation frequencies from a target oscillation frequency is obtained. Then, a DC voltage corresponding to this frequency deviation is applied to a piezoelectric mother substrate, and a frequency adjustment is performed by varying the degree of polarization.
In recent years, in ceramic oscillators, it is required to control the oscillation frequency with a higher degree of accuracy. Specifically, the required accuracy of the oscillation frequency is within 0.1%.
However, in the conventional method (1) for adjusting the thickness of a piezoelectric mother substrate or that of a resonant electrode, it is necessary to control the thickness of the piezoelectric mother substrate or the resonant electrode in units of 1/10 xcexcm in order to control the accuracy of the oscillation frequency within 0.1%. However, the working with such a thickness accuracy entails a very high cost, which significantly impairs a ceramic oscillator""s advantage that it is less expensive than quartz crystal.
On the other hand, the method (2) for adjusting the frequency constant to adjust the oscillation frequency of a ceramic oscillator, does not require a high-accuracy working as described above. However, in the method set forth in the Japanese Unexamined Patent Application Publication No. 7-106893, the frequencies of ceramic oscillators each having an outer package applied are measured, and the ceramic oscillators are sorted out by the comparison between the frequencies and the target frequency range. It is, therefore, necessary to further perform a secondary polarization processing for the ceramic resonators falling outside the target range so as to fall within a predetermined range, by further applying a DC voltage. This raises a problem in that the number of processes increases, resulting in an increased manufacturing cost. In addition, since this method requires complicated processes, it causes another problem of taking a long time for the manufacturing process.
Accordingly, it is an object of the present invention to solve the above-described problems caused by the conventional arts, and to provide a manufacturing method that is capable of adjusting the frequency by adjusting the frequency constant through relatively a simple process, for obtaining ceramic oscillators from a piezoelectric mother substrate, and that allows inexpensive ceramic oscillators, of which frequencies are controlled with a high accuracy, to be achieved.
In order to achieve the above-described object, the present invention provides a method for manufacturing a ceramic oscillator, including the step of performing polarization processing for a mother substrate; the step of forming electrodes on the mother substrate in discrete ceramic oscillator units; and the step of cutting the mother substrate into discrete ceramic oscillator units, and thereby obtaining discrete ceramic oscillators. Herein, the step of performing polarization processing for the mother substrate is executed by finishing the application of a high DC voltage when the antiresonant frequency fa of the mother substrate in a thickness vibration mode is measured while the voltage is applied to the mother substrate, and the antiresonant frequency fa which is being measured has reached a target value which is the antiresonant frequency of the mother substrate during polarization corresponding to a target oscillation frequency of the ceramic oscillator as a finished product.
In the manufacturing method for a ceramic oscillator in accordance with the present invention, correlated data which are used for determining the above-described target value of the antiresonant frequency of the mother substrate during polarization preferably include first correlated data exhibiting the correlation between the oscillation frequency of the ceramic oscillator which has ultimately been obtained and the antiresonant frequency of the mother substrate at room temperature, and second correlated data exhibiting the correlation between the antiresonant frequency fa of the mother substrate at room temperature and the antiresonant frequency fa of the mother substrate during polarization.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.